Add support implicit conversion between types

[ilya-stromberg]

On Friday, 6 September 2013 at 13:25:37 UTC, Flamaros wrote:
> It's difficult to never forget the "explicit" keyword in c++, 
> and this little mistake can make you loose a lot of time.
>
> I prefer to only have explicit conversions with cast or 
> constructors calls, that all make the code easier to understand.

Yes. I sugest keep current D bechavior by default, but add 
"implicit" keyword.
It allows to add implicit cast only if it SAFE.